No longer using ‘strong’ in property declarations

I realized that using ‘strong’ in place of ‘retain’ when not using ARC is dumb since ‘retain’ is more specific (non-ARC ‘retain’ and ‘copy’ both map to ‘strong’ in ARC). In light of consistency, I'm deciding to stick to the old names since they map properly and automatically to the proper ARC qualifiers, and since I would have to use ‘copy’ explicitly to get its semantics.

Also:
• ‘nonatomic’ always comes first. It's just cooler that way.
Cleaned up drawing/scaling

• ArenaVisRep now takes PPI into account for its drawing
	• PPI is taken as in the initializer, and also accessible by property
	• SceneView#drawContentLayerWithin: no longer scales, now only translates
	• SceneView#drawContentLayerWithin:, of course, now only translates by pixels, not inches.
• ArenaVisRep#draw is now ArenaVisRep#drawWithin:, taking an draw-update-area rect
	• SceneView#drawContentLayerWithin: also now calculates the translated draw rect and hands it off to ArenaVisRep#drawWithin:
• SceneView now fills with a clearingColor (just black) before each draw, to prevent unwanted overdraw (for now)
• ArenaVisRep now draw its own background, currently a white fill (but more soon)

Minor:
• ArenaVisRep#ringColor changed to a readonly property
• All retain-forever static accessors (the color “constants”) are now assign properties, since they really aren't going anywhere.
• static vars for screenPPI & screenPPIRetinaDisregarded moved into the methods (easier to browse, but harder to debug I know)
• debug line in screenPPI fixed to report screenPPI, not screenPPIRetinaDisregarded
Minor: SDTLog now prints a newline and tab before the and the given string (after the timestamp and code location)
Minor: SDTLog character spacing improved

Although this may be hard to spot in the diff, there are now zero-width spaces (U+200B) between the tiny superscript characters and the inserted data (filename, line number, class name, instance pointer, method name).

This makes it much much easier to select because OS smart selections (double-click, shift-left/right, etc.) will stop at the spaces and exclude the superscript chars.